The present invention relates to wavelength division multiplexing (WDM) optical networks and, more particularly, to measurement of spectral fragmentation in the WDM networks.
In wavelength division multiplexing (WDM) optical networks, upon an arrival of a demand requesting a line rate between end nodes, an optical channel is established by allocating finite amount of spectrum on all fibers along the route. If an intermediate node along the route does not support wavelength conversion capability, then the channel must follow the wavelength continuity constraint, which is defined as an allocation of the same center wavelength to the channel, and the spectral continuity constraint, which is defined as an allocation of the same amount of spectrum to the channel in the ingress and egress fibers of a node. To support multiple such channels over a fiber, the spectral conflict constraint must be satisfied, which is defined as an allocation of non-overlapping spectrum to the channels routes over the same fiber.
Conventionally to address interoperability issues, the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) has standardized fixed channel spacing [1]. The network that follows the ITU-T standard is referred to as fixed grid network as shown in FIG. 1(a). Fixed grid networks may not optimize spectral efficiency while supporting line rates with heterogeneous granularity for ever increasing bandwidth demands. Recently, flexible grid network architecture (as shown in FIG. 1(b)) is introduced in which flexible amount of spectrum is assigned to channels based on the requirements of requested bandwidth, transmission reach, and offered modulation formats. Flexible grid networks highly optimize network spectral efficiency. However, dynamic arrival and departure of channels with heterogeneous spectrum requirements causes fragmentations in spectrum (as shown in FIG. 2), and the network can no longer be in its optimal state. The state of fragmented spectrum in a network is referred to as network fragmentation. Network fragmentation is a serious issue in fixed and flexible grid networks. Spectral fragmentation can block a connection in spite of the availability of sufficient amount of spectrum for the connection and deteriorate the network performance.
To restore a network in its optimal state and enhance the blocking performance, several network defragmentation schemes are investigated [2][3][4]. However, in order to make timely judgments on when to defragment the network and to verify the effectiveness of network defragmentation or resource provisioning solutions, there is a need to accurately measure the network state. Thus, the problem is how to quantify the fragmentation in a network. The problem is formally defined as follows.
We are given an optical network topology G(V, E), where V is a set of reconfigurable optical add-drop multiplexer (ROADM) nodes and E is a set of fibers connecting ROADM nodes. The network supports a set of line rates L. The required spectral width for each line rate is H1 GHz for a line rate l. The network offers total Z GHz of spectrum to support the network traffic. A connection demand requesting a line rate l between a set of nodes arrive with probability Ql. The problem is how to measure spectral fragmentation in the network.
In [5], a Spectrum Compactness parameter is introduced to quantify fragmentation over a fiber based on the occupied and available spectrum slots. In [6], Utilization Entropy parameter is introduced to evaluate fragmentation over a fiber based on the number of changes in the states of spectrum. Both of these parameters do not consider fiber correlations while measuring network fragmentation.